The objectives of this proposal are to elucidate the molecular mechanism that ensures selectivity of arrestin towards phosphorylated photo-activated rhodopsin and to establish the link between the timing of arrestin binding and the kinetics of photoresponse recovery. Using site-directed mutagenesis, amino acid residues in arrestin that directly interact with rhodopsin, as well as amino acids that participate in individual intramolecular regulatory interactions, are to be identified. On the basis of these data, an approximation of the three-dimensional organization of the arrestin molecule is to be defined. In particular, a mutant form of arrestin that appears to be constitutively active has been produced. This mutant arrestin binds with high affinity to photo-activated rhodopsin regardless of the phosphorylation state of the rhodopsin. It is to be expressed in transgenic mouse photoreceptors in an arrestin null background. Physiological analyses of these mice are to be performed to determine whether the rate-limiting step in photoreceptor deactivation following a light flash is inactivation of photoactivated rhodopsin or inactivation of transducin-activated phosphodiesterase. The characterization of the functional consequences of expressing mutant arrestins in vivo is expected to pave the way for future use of such mutants for gene therapy of congenital retinal disorders.